This invention relates to an automatic transmission and, more particularly, to an automatic transmission which provides appropriate gear ratios for each gear shift applied to vehicles.
As a prior art, an automatic transmission is disclosed in a Japan Patent Laid-open Publication 52-149562 (published on Dec. 12, 1977). This prior art discloses an automatic transmission in which three lines of planetary gears having a ring gear, a carrier, and a sun gear are disposed in series. The automatic transmission is capable of switching in six forward shifts and one reverse shift with five frictional engaging elements including two clutch elements and three brake elements actuated by an outer force.
However, with the structure of the automatic transmission employed in the above mentioned prior art, the gear ratio for a reverse shift becomes large when setting up the optimum gear ratio step for forward gears. When enlarging the gear ratio of the second forward speed, the gear ratio of the fifth forward speed is undesirably reduced. When reducing the gear ratio of the second forward speed, other problems are caused, e.g., the gear ratio of the fifth forward speed is enlarged, but the shifting from fifth forward speed to the sixth forward speed becomes undesirable and the pinion rpm at the sixth forward speed is increased.
Accordingly, an object of this invention is to provide an automatic transmission capable of obtaining the appropriate gear ratio with six forward shifts and one reverse shift.
To solve the foregoing problems, the automatic transmission of this invention is provided with the following technical features which include: an input shaft, an output shaft, a double pinion planetary gear in the first line, of which one of a sun gear or a carrier is connected to the input shaft; a single planetary gear in the second line, of which a ring gear is connected or is selectively connectable to a ring gear of the double pinion planetary gear in the first line; a planetary gear unit provided in series including a single planetary gear in a third line, of which a ring gear is connected to the ring gear of the double pinion planetary gear in the first line; a single planetary gear in the fourth line, of which a ring gear is connected to a carrier of the single planetary gear in the third line and of which a sun gear is connected to a sun gear of the single planetary gear in the third line; a first shaft element connected to the ring gear of the single planetary gear in the third line; a second shaft element connected to the carrier of the single planetary gear in the third line; a third shaft element connected to a carrier of the single planetary gear in the fourth line and to the output shaft; and a fourth shaft element connected to the sun gear of the single planetary gear in the third line. A first clutch element C1 is capable of selectively connecting the input shaft and the fourth shaft element of the planetary gear unit. A second clutch element C2 is capable of connecting the input shaft and the carrier of the single planetary gear in the second line. A first brake element B1 is capable of selectively fixing the first shaft element of the planetary gear unit, and a second brake element B2 is capable of selectively fixing the second shaft element of the planetary gear unit. A third brake element B3 is capable of selectively fixing the sun gear or the other carrier of the double pinion planetary gear in the first line.
Another feature of the automatic transmission of this invention involves the second clutch element being capable of selectively interconnecting the first shaft element and the ring gear of the single planetary gear in the second line by normally interconnecting the input shaft and the carrier of the single planetary gear in the second line instead of the second clutch element which is capable of selectively connecting the input shaft and the carrier of the single planetary gear in the second line.
A further feature of the automatic transmission of this invention involves the second clutch element being capable of selectively interconnecting the fourth shaft element and the sun gear of the single planetary gear in the second line by normally interconnecting the input shaft and the carrier of the single planetary gear in the second line instead of the second clutch element which is capable of selectively connecting the input shaft and the carrier of the single planetary gear in the second line.
The automatic transmission enables six speed forward shifts and one reverse shift to be established by switching the combinations of five frictional engagement elements including the first clutch element C1, the second clutch element C2, the first brake element B1, the second brake element B2, and the third brake element B3. Since the ratio of the number of the teeth between the ring gear and the sun gear of the planetary gear in the first line depends on the size of the gear ratio of the planetary gear in the first line being fixed, the gear ratio of the forwarding shifts and the reverse shift is modified to the appropriate gear ratio by adopting the double pinion planetary gear for the planetary gear in the first line. In addition, by adding the single planetary gear in the second line, the gear ratio which is not modified by the double pinion planetary gear in the first line is set appropriately. The automatic transmission having the features explained above can provide appropriate gear ratios for each shift of six forward and one reverse shifts.